Structure and properties of rat thymus deoxyribonucleoprotein II. Effects of enzymes and other agents on electric birefringence properties

Abstract

In a study of the structure of DNP from rat thymus, the role of the constituent DNA and protein has been investigated by adding DNA ase and proteolytic enzymes. Also, the effect of RNA ase has been studied. Preliminary studies have been made on DNA prepared from the DNP and direct from rat thymus. Attempts have been made to identify the type of interlink between DNP molecules by adding EDTA and urea. The method used is that of electric birefringence: this gives information about the electrical and optical properties of the molecules. From the decay of birefringence on removal of the electric field, the lengths of the molecules can be calculated. The action of proteolytic enzymes and of DNA ase shows that their respective substrates are attacked within the DNP molecules. The effects of proteolytic enzymes indicate also that ( a ) protein does not link the DNA molecules lengthways unless it does so in such a way as to form loops ( b ) the DNP is more rapidly degraded when kept in suspension in 0.15 M NaCl than when in solution and ( c ) limited enzyme action resembles that of ageing. The DNA ase results depend on DNP concentration; this is explained on the basis that the DNP particles consist of lateral aggregates which, on dilution, either dissociate laterally (at least partially) or become more labile. RNA ase produces only a very small change in birefringence properties. The changes produced by added EDTA are attributed merely to salt effects, indicating that if metal links are present, they are not important within DNP aggregates. The effect of added urea, which resembles in some respects that of proteolytic enzymes, cannot be interpreted unambiguously as showing that hydrogen bonds are important in the original structure. With DNA , ageing effects and the action of DNA ase depend on the concentration of the DNA .